In connection with vehicles which use internal combustion engines, there is a general desire for reducing the exhaust of harmful substances in the exhaust gas from the engine. These harmful substances are primarily pollutants in the form of nitrogen oxide compounds (NO.sub.x), hydrocarbon compounds (HC) and carbon monoxide (CO).
In modern motor vehicles with gasoline engines, purification of the exhaust gases is normally carried out by means of an exhaust gas catalyzer in the exhaust gas system, through which the exhaust gases flow. In a so-called known three-way catalyzer, the main portion of the abovementioned harmful substances is eliminated by means of known catalytic reactions.
Although modern three-way catalyzers normally have a very high degree of purification, which to a very large extent limits the exhausts of harmful pollutants into the atmosphere, there are presently demands for even lower exhausts of harmful substances from motor vehicles. These demands arise, among other things, from increasingly restrictive legislation in various countries, with demands for extremely low exhausts of NO.sub.x, CO and HC-compounds. Modern conventional three-way catalyzers also have a drawback in that they have a relatively low degree of purification when cold-starting an engine, i.e. the catalyzer is not able to provide optimal purification from the start of the engine until the catalyzer has reached its so-called "light-off" temperature. In modern vehicles, the light-off temperature is from about 200.degree. to 350.degree. , and can be defined as the temperature at which the catalyzer provides 50% purification of a certain harmful component in the exhaust gases.
The initial period of time which elapses until the lightoff temperature is reached is, in modern vehicles, normally from about 30 to 90 seconds. During this time, a certain amount of harmful components in the exhaust gases will thus pass through the exhaust system of the internal combustion engine and out into the surrounding atmosphere. In order to minimize these harmful exhausts, there is thus a need to minimize the time which elapses until the catalyzer reaches its light-off temperature. This can be obtained, for example, by developing new catalyzer materials which exhibit constantly lower ignition temperatures, and by developing methods for rapid heating of the catalyzer. A reduction of the harmful emissions from an internal combustion engine can also be obtained by utilizing a HC-adsorbent which is arranged in the exhaust pipe in order to adsorb the undesired components in the exhaust gases, until that point in time when the catalyzer has reached its light-off temperature.
A previously known method for reducing the harmful exhausts from an internal combustion engine is to combine its regular catalyzer with another, electrically heatable, starting catalyzer which is placed upstream of the regular catalyzer. Since the starting catalyzer is heated rapidly using electricity, its catalytic deposition quickly becomes active, thus causing a significant purification of the exhaust gases at low idling flows. The exhaust gases which flow therethrough are heated further, which in turn shortens the heating time of the regular catalyzer. A device which comprises such a heatable starting catalyzer, which provides an improved purifying effect when cold-starting the engine, is previously known from International Publication No. WO 89/10470.
A drawback associated with electrically heatable starting catalyzers is that they require a very large current supply. This creates high demands on the electrical system of the vehicle, as well as higher costs since, for example, an extra starting battery is normally necessary in order to power the starting catalyzer. Another problem with such systems is the amount of time during which they can be used. Since the starting catalyzer ages relatively rapidly, degraded performance sets in quickly.
Another arrangement for the reduction of harmful exhausts from an internal combustion engine disclosed in European Patent No. 0,422,432, and comprises a combustion chamber in which combustible components in the exhaust gases can be ignited in order to raise the temperature of the gases which pass the catalyzer. Thus, the temperature of the catalyzer itself can be raised more quickly than normal.
A drawback with such an arrangement, however, is that the amount of unused components in the form of hydrocarbons, carbon monoxide, hydrogen and oxygen is often insufficient to cause rapid and reliable heating of the catalyzer. Also, an extra component in the form of an ignition means in order to ignite a gas compound becomes necessary.
Other devices intended to reduce harmful exhausts are disclosed in International Publication Nos. WO 92/22734 and WO 93/07365. These documents disclose systems with an after-burner in the exhaust system, in which a mixture of exhaust gases which reach a certain ignition zone in the after-burner can be ignited using an ignition device. In order to obtain the desired concentrations in the mixture of exhaust gases, the mixture of fuel/air to the engine is controlled so that a predetermined surplus of hydrogen is obtained in the gas mixture while secondary air is supplied using a pump.
A drawback in this latter arrangement is that it requires a separate ignition device in order to ignite the gas mixture, which raises the cost. Additionally, a special air-pump is necessary for the supply of secondary air, which adds further to the cost. A further drawback is that this device requires a relatively large amount of space in the vehicle.
A further method of reducing the emissions from an internal combustion engine is to utilize a so-called HC-adsorbent or HC-trap. A device which comprises an HC-adsorbent is disclosed in Swedish patent application no. 9302407-3. To be more precise, this arrangement comprises an HC-adsorbent which is arranged in the exhaust pipe, upstream of an electrically heatable starting catalyzer and a conventional three-way catalyzer. The HC-adsorbent is arranged to adsorb, i.e. "gather" HC-compounds in the exhaust gases before the three-way catalyzer has reached its light-off temperature. When the temperature in the flow of exhaust gases reaches a certain temperature, the HCadsorbent will emit (desorb) those HC-compounds which have previously been gathered.
A substantial drawback with previously known HC-adsorbents is that they age due to the temperature of the exhaust gases.
This ageing process causes a lowered temperature limit for desorption of the HC-adsorbent. This causes the HC-adsorbent to emit its HC-compounds before the ordinary catalyzer has ignited.
According to these prior systems, a reduction in harmful exhausts is realized by shortening the time which elapses until the catalyzer reaches its lightoff temperature. Using previously known technology, the light-off temperature can be reached within about 10 to 15 seconds following a cold-start. As regards exhausts of HC-compounds, the concentration of these substances is very high approximately 2 to 4 seconds following a cold-start, which is of major significance in those systems which do not utilize an HC-adsorbent or corresponding technology. In such systems, there is only a limited reduction of the exhausts of HC-compounds in connection with cold-starts, although they significantly shorten the time which elapses until the catalyzer reaches its light-off temperature.
An object of the present invention is thus to provide an improved device for the purification of harmful components, in particular HC-compounds in the exhaust gases of an internal combustion engine, without using an HC-adsorbent in the exhaust pipe.